2. Field of the Invention
The invention relates to novel copolymer products, and to novel processes for producing such products. More specifically, the invention relates to blends of two copolymers, particularly ethylene propylene copolymers (EPM) or terpolymers (EPDM), one having a relatively narrow molecular weight distribution (MWD), and one having a relatively broader MWD, as well as to processes for producing such blends. The invention further relates to the use of such blends in lube oil compositions, and in elastomer preparations.
3. Description of Background Information and Relevant Materials
For convenience, certain terms that are repeated throughout the present specification are defined below:
a. Inter-CD defines the compositional variation, in terms of ethylene content, among polymer chains. It is expressed as the minimum deviation (analogous to a standard deviation) in terms of weight percent ethylene from the average ethylene composition for a given copolymer sample needed to include a given weight percent of the total copolymer sample which is obtained by excluding equal weight fractions from both ends of the distribution. The deviation need not be symmetrical. When expressed as a single number for example 15% Inter-CD, it shall mean the larger of the positive or negative deviations. For example, for a Gaussian compositional distribution, 95.5% of the polymer is within 20 wt. % ethylene of the mean if the standard deviation is 10%. The Inter-CD for 95.5 wt. % of the polymer is 20 wt. % ethylene for such a sample.
b. Intra-CD is the compositional variation, in terms of ethylene, within a copolymer chain. It is expressed as the minimum difference in weight (wt.) % ethylene that exists between two portions of a single copolymer chain, each portion comprising at least 5 weight % of the chain.
c. Molecular weight distribution (MWD) is a measure of the range of molecular weights within a given copolymer sample. It is characterized in terms of at least one of the ratios of weight average to number average molecular weight, M.sub.w /M.sub.n and Z average to weight average molecular weight, M.sub.z /M.sub.w, where: ##EQU1## Ni is the number of molecules of weight Mi.
d. Viscosity Index (V.I.) is the ability of a lubricating oil to accommodate increases in temperature with a minimum decrease in viscosity. The greater this ability, the higher the V.I.
Ethylene-propylene copolymers, particularly elastomers, are important commercial products. Two basic types of ethylene-propylene copolymers are commercially available. Ethylene-propylene copolymers (EPM) are saturated compounds requiring vulcanization with free radical generators such as organic peroxides. Ethylene-propylene terpolymers (EPDM) contain a small amount of non-conjugated diolefin, such as dicyclopentadiene; 1,4-hexadiene or ethylidene norbornene, which provides sufficient unsaturation to permit vulcanization with sulfur. Such polymers that include at least two monomers, e.g., EPM and EPDM, will hereinafter be collectively referred to as copolymers.
EPM and EPDM have outstanding resistance to weathering, good heat aging properties and the ability to be compounded with large quantities of fillers and plasticizers resulting in low cost compounds which are particularly useful in automotive and industrial mechanical goods applications. Typical automotive uses are tire sidewalls, inner tubes, radiator and heater hose, vacuum tubing, weather stripping and sponge doorseals and Viscosity Index (V.I.) improvers for lubricating oil compositions. Typical mechanical goods uses are for appliance, industrial and garden hoses, both molded and extruded sponge parts, gaskets and seals and conveyor belt covers. These copolymers also find use in adhesives, appliance parts, hoses and gaskets, wire and cable insulation, and plastics blends.
The efficiency of peroxide curing depends on composition. As indicated at column 2, lines 31-40 of COZEWITH et al., the "chemical" crosslinks per peroxide molecule increases as the ethylene level increases; ethylene content also affects rheological and processing properties, and the crystallinity present at very high ethylene contents may hinder processibility.
As more fully described at column 2, lines 40-49 of COZEWITH et al., milling behavior of EPM or EPDM copolymers varies radically with MWD; among the properties affected by variation in MWD is the shear rate dependence of viscosity.
A motor oil should not be too viscous at low temperatures so as to avoid serious frictional losses, facilitate cold starting and provide free oil circulation right from engine startup. On the other hand, it should not be too thin at working temperatures so as to avoid excessive engine wear and excessive oil consumption. It is most desirable to employ a lubricating oil which experiences the least viscosity change with changes in temperature
The ability of a lubricating oil to accommodate increases in temperature with a minimum decrease in viscosity is indicated by its Viscosity Index (V.I.). The greater this ability, the higher the V.I.
EPDM or EPM polymers, or, more generally, ethylene-(C.sub.3 -C.sub.18) alpha-olefin copolymers, serve to impart desirable viscosity-temperature characteristics to compositions, including lubricating oils. Lubricating oil compositions which use EPM or EPDM copolymers or, more generally, ethylene-(C.sub.3 -C.sub.18) alpha-olefin copolymers, as V.I. improvers are well known. These additives are designed to modify the lubricating oil so that changes in viscosity occurring with variations in temperature are minimized as much as possible. Lubricating oils containing such polymeric additives essentially maintain their viscosity at higher temperatures while at the same time maintaining desirable low viscosity fluidity at engine startup temperatures.
Two important properties (although not the only required properties as is known) of these additives relate to low temperature performance and shear stability. Low temperature performance relates to maintaining low viscosity at very low temperatures, while shear stability relates to the resistance of the polymeric additives to being broken down into smaller chains.
As can be seen from the above, based on their respective properties, EPM and EPDM find many, varied uses. It is known that the properties of such copolymers which make them useful in a particular application are, in turn, determined by their composition and structure. For example, the ultimate properties of an EPM or EPDM copolymer are determined by such factors as composition, compositional distribution, sequence distribution, molecular weight, and molecular weight distribution (MWD).
It is well known that the breadth of the MWD can be characterized by the ratios of various molecular weight averages. One of such averages is the ratio of weight to number average molecular weight (M.sub.w /M.sub.n). Another of the ratios is the Z average molecular weight to weight average molecular weight (MM.sub.z /M.sub.w).
Copolymers of ethylene and at least one other alpha-olefin monomer, including EPM and EPDM polymers, which are intramolecularly heterogeneous and intermolecularly homogenous, and which have a narrow MWD, characterized as at least one of M.sub.w /M.sub.n less than 2 and M.sub.z /M.sub.w and less than 1.8, have improved properties in lubricating oil. Such copolymers are disclosed in COZEWITH et al., U.S. Pat. No. 4,540,753, which is incorporated herein by reference. For convenience, such polymers are hereinafter referred to as narrow MWD copolymers. Copolymers having MWD with at least one of M.sub.w /M.sub.n greater than or equal to 2 or M.sub.z /M.sub.w greater than or equal to 1.8 are hereinafter referred to as broad MWD copolymers.
It is generally recognized that the cure rate and physical properties of copolymers of ethylene and at least one other alpha-olefin monomer are improved as MWD is narrowed. Narrow MWD polymers have superior cure and tensile strength characteristics over such polymers having broader MWD. However, the advantages in physical properties gained from having a narrow MWD are offset by the poorer processability of such materials. They are often difficult to extrude, mill, or calendar.
As to milling behavior of EPM or EPDM copolymers, this property varies radically with MWD. Narrow MWD copolymers crumble on a mill, whereas broad MWD materials will band under conditions encountered in normal processing operations. At the shear rates encountered in processing equipment, broader MWD copolymer has a substantially lower viscosity than narrower MWD polymer of the same weight average molecular weight.
U.S. Pat. No. 3,681,306 to WEHNER, disclosed in the COZEWITH et al. patent, is drawn to a process for producing ethylene/higher alpha-olefin copolymers in at least 2 consecutive reactor stages. The preferred polymers produced are ethylene/propylene/non-conjugated diene terpolymers, the diene having only one polymerizable double bond. The preferred process uses one tubular reactor followed by one pot reactor; however, it is also disclosed that one tubular reactor can be operated at different reaction conditions to simulate two stages. As is seen from column 2, lines 14-20, the process disclosed produces polymers of broader MWD, as measured by the ratio of the weight average to number average molecular weights M.sub.w /M.sub.n) than those made in a single stage reactor. Although intermediate polymer produced from the first (pipeline) reactor is disclosed as having a ratio of M.sub.w /M.sub.n) of about 2, as disclosed in column 5, lines 54-57, the final polymers produced have an M.sub.w /M.sub.n) of 2.4 to 5.
U.S. Pat. No. 3,380,978 to RYAN et al. discloses a process for producing homopolymers and copolymers prepared from alpha-olefins containing 2-18 carbon atoms. In the process of this patent, a short holdup tubular reactor is employed in a first stage to produce a high molecular weight fraction having a broad molecular weight distribution, whereafter the polymer produced and remaining reaction mixture are passed directly into a second stage; in this stage, a longer holdup, constant environment autoclave reactor is employed to form a narrower molecular weight distribution fraction. It is noted that the broader MWD polymer is produced in the tubular reactor. Further, no numerical values for MWD or for molecular weight are given.
U.S. Pat. No. 3,035,040 to FINDLAY discloses a process for producing olefin polymers and copolymers from such monomers as ethylene and propylene. At least a portion of the monomers are first polymerized in the presence of a catalyst in dilute phase in a stream-line flow reaction zone. The partially polymerized admixture is subjected to further polymerization in an agitated zone to substantially complete the polymerization reaction. The first reaction zone can be an elongated tube. The second reaction zone can be an agitated tank-type reaction zone, preferably a series of stirred reactors. Polymer product can be recovered from the agitated zone effluent, with unreacted olefin and diluent being recycled to the tubular zone. There is no disclosure of difference in MWD or molecular weight between the polymers produced in the two reactors.
U.S. Pat. No. 3,884,993 to GROS discloses a process for producing EPM and EPDM rubbers which comprises producing low molecular weight and high molecular weight fractions in separate reactors, in parallel, and blending the products. Here, too, there is no mention of MWD.